In an active-matrix display device such as a liquid crystal display, a thin-film transistor device referred to as a thin-film transistor (TFT) has been used. In the display device, the TFT is used as a switching device for selecting a pixel or a driving transistor for driving the pixel.
In recent years, organic EL displays using electroluminescence (EL) of an organic material have been attracting attention as a type of next-generation flat panel display replacing liquid crystal displays.
Unlike the voltage-driven liquid crystal display, the organic EL display is a current-driven display device. Accordingly, there is an urgent need for development of a thin-film transistor device having excellent on/off-characteristics as a driving circuit for the active-matrix display device. A TFT has a structure in which a gate electrode, a semiconductor layer (channel layer), a source electrode, and a drain electrode are formed on a substrate. The channel layer is typically a silicon thin film.
Moreover, display devices are demanded to increase a screen size and reduce a cost. In general, because of easy cost reduction, bottom-gate TFTs are used. A bottom-gate TFT has a gate electrode formed closer to a substrate more than a channel layer is.
The bottom-gate TFTs are classified into two major categories: channel-etching TFTs in which a channel layer is etched, and channel-stopper TFTs in which a channel layer is prevented from being etched.
In comparison to the channel-stopper TFTs, the channel-etching TFTs can decrease steps of photolithography, thereby reducing a manufacturing cost.
On the other hand, the channel-stopper TFTs can prevent the channel layer from being damaged by etching, thereby suppressing the increase of characteristic variations on the surface of the substrate. Moreover, a channel layer of the channel-stopper TFT can be manufactured to be thinner than that of the channel-stopper TFT. As a result, the channel-stopper TFTs can reduce parasitic resistance components to improve the turn-On characteristics. The channel-stopper TFTs are therefore advantageous for high resolution.
Therefore, the channel-stopper TFTs are suitable as, for example, driving transistors in current-driven organic EL display devices having organic EL elements. Even if a manufacturing cost of the channel-stopper TFTs is higher than that of the channel-etching TFTs, channel-stopper TFTs have been attempted to be applied to pixel circuits in organic EL display devices.
For example, Patent Literature 1 discloses a channel-stopper TFT in which a gate electrode, a gate insulating film, a first semiconductor film composed of an n-type microcrystalline silicon, a second semiconductor film composed of amorphous silicon, and a back channel stopper insulating film are sequentially formed on a substrate in order to suppress back channel effects caused by fixed charges of a channel stopper film.